1. Field of the Invention
The present invention relates to an indexing device for turret lathes and other machine tools, and more particularly to an indexing device which produces a prescribed intermittent motion of an output shaft by engagement between the taper rib of a globoidal cam and a cam follower fixed on the indexing device.
2. Description of the Prior Art
Machines such as a turret lathe comprise an indexing device to which are fastened, in advance, a variety of cutting tools necessary for successive machining processes, and which during machining is suitably rotated so that the cutting tools may be brought sequentially into cutting position for required processing. Some known indexing devices of this type utilize a globoidal cam.
Generally, in the indexing device having the globoidal cam, a plurality of cam followers are fixed radially at suitable intervals along the outer periphery of the indexing disk integral with the output shaft, and these cam followers sequentially engage with the taper rib on the globoidal cam which is integral with the input shaft. The two cam followers immediately adjoining the cam follower in engagement with the cam abut against the right and left lateral sides of the taper rib respectively so that no backlash occurs between the globoidal cam and the cam followers; that is to say, this cam is a positive motion cam.
The taper rib of the globoidal cam defines an index portion in which it undergoes a displacement in the direction of the input shaft in response to an angle of rotation of the input shaft, and a dwell portion in which it undergoes no displacement in the direction of the input shaft in response to the rotation of the input shaft; and these portions are determined so that the desired cam profile can be obtained. Accordingly, when the cam follower is in engagement with the index portion, the output shaft rotates a predetermined angle in response to the rotation of the input shaft so that one of the cutting tools may be brought into cutting position; and when in engagement with the dwell portion, the output shaft does not rotate despite the rotation of the input shaft so that the cutting tool which has been brought into cutting position may remain in the same cutting position. In other words, when the cam follower is in engagement with the dwell portion, the output shaft is secured firmly at the desired position without any locking means such as a curvic coupling, gear coupling or the like.
The input shaft is driven by a motor having speed control, and when during machining the cam follower is in engagement with the dwell portion, the motor comes to rest.
However, in the indexing device of the type described, if an excessive load such as an impact is unexpectedly applied to one of the input and output shafts when the cam follower is in engagement with the dwell portion of the taper rib, the excessive load acts on the interface between the taper rib and the cam follower, which may cause permanent deformation such as an imprint caused by a pressure on the contact surfaces, or a warp in the cam follower; as a result the indexing device can not attain the desired accuracy by the permanent deformations.